The invention disclosed herein pertains generally to hydraulic control systems for distributing fluid to a plurality of fluid actuated devices, and more particularly to a hydraulic control system which ensures that a particular fluid actuated device receives an adequate supply of fluid even when being used in conjunction with one or more of the other of the plurality of fluid actuated devices.
It is frequently necessary for a single hydraulic pump to provide sufficient hydraulic fluid to operate a plurality of fluid actuated devices. In the past it has been common to utilize fixed displacement pumps in combination with open center control valves to distribute the fluid to the various fluid actuated devices. Such systems have several drawbacks including the fact that they are energy inefficient because when the fluid actuated devices utilize only a portion of the output of the fixed displacement pump, the remainder of the pump power is wasted.
To overcome the deficiencies associated with hydraulic control systems which utilize fixed displacement pumps, variable displacement pumps have been used in combination with closed center control valves to achieve better system efficiency. In some applications the variable displacement pumps have been provided with pressure compensated controls, and in other applications variable displacement pumps in combination with a closed center control valve have been utilized in a load sensitive system. The former systems have usually been characterized by large pressure drops which results in a very inefficient use of energy. In the latter system, when a load is being lowered under the influence of gravity the load signal may reverse, resulting in diminished pump output rather than the desired increase in pump output.
The hydraulic control system disclosed in U.S. Pat. No. 3,788,077 to Johnson et al includes a variable displacement pump, an open center type of control valve, and a fluid actuated displacement control mechanism for the pump. A control pressure signal indicative of pump output pressure is developed in the control valve, and this control signal is communicated to the displacement control mechanism to regulate pump displacement. Systems of this type generally lack the responsiveness and sensitivity needed for the control of many types of fluid actuated devices.
The hydraulic control system disclosed in U.S. Pat. No. 4,197,705 to Westveer, which is hereby incorporated by reference, overcomes many of the aforementioned problems. This hydraulic control system includes a variable displacement pump which supplies fluid to a control valve which includes a plurality of control valves, each of which valves regulates the flow of fluid to a fluid actuated device. Each of the valves includes a relatively small, variable orifice in its center. A relatively small fluid flow from the variable displacement pump is supplied to the variable orifice of the first valve, and the output from each variable orifice of each valve is then directed to the input of the variable orifice of each succeeding valve. The resulting pressure signal sensed at a point downstream from the last of the variable orifices is referred to as a center core signal and constitutes a control signal. This control signal is communicated to a fluid actuated displacement control device which regulates the displacement of the variable displacement pump in response to the magnitude of the control signal. As the plunger of any one of the valves is moved from the neutral position toward an operating position, indicating the need for a greater supply of fluid to that valve, the size of the variable orifice of that valve is reduced, resulting in a decrease in the magnitude of the control signal. As the plungers of the other valves are moved from their neutral positions toward their operating positions, the magnitude of the control signal is further reduced. The displacement control device is such that a decrease in the magnitude of the control signal results in a corresponding increase in the displacement of the variable displacement pump.
While the hydraulic control system disclosed in U.S. Pat. No. 4,197,705 to Westveer is energy efficient and sufficiently sensitive and responsive for the control of many types of fluid actuated devices, neither this hydraulic control system nor any of the other above-mentioned hydraulic control systems is capable of ensuring that one particular fluid actuated device will be supplied with an adequate flow of fluid when being used in conjunction with one or more other fluid actuated devices.